1. Field of the Invention
The present invention relates to an operation microscope and an observation prism in an ophthalmologic field. In particular, the present invention relates to an operation microscope which is provided with a front lens for observing a fundus of an eye to be operated and an observation prism disposed near the front lens to observe fundus and its surroundings of the eye to be operated during a vitreous body operation or the like, and the, observation prism used in the operation microscope.
2. Description of the Related Art
An operation in an ophthalmologic field is generally conducted with a microscope observation. An example of an operation microscope used in the ophthalmologic field is disclosed in JP 2003-062003 A. The operation microscope disclosed therein is constructed such that the front lens is removably inserted between (anterior focal position of) an objective lens and the eye to be operated. The front lens has refracting power of about 30 diopters (D) to 50 diopters (D) and is disposed to condense illumination light, thereby guiding the illumination light to the interior of the eye to be operated. Using the front lens enables an operation in a state where operational instruments are held with both hands.
An observation prism for observing fundus and its surroundings of the eye to be operated is rotatably provided near the front lens of the operation microscope described in JP 2003-062003 A. A bottom surface and an oblique surface of the observation prism each are formed flat.
FIGS. 8, 9A, and 9B are schematic views showing a part of an optical system included in the above-mentioned conventional operation microscope. FIG. 8 is a side view showing an imaging state of an entrance pupil of an observation optical system in the case where the fundus and its surroundings of the eye to be operated are being observed using the above-mentioned observation prism. FIG. 9A is a partially enlarged side view showing the front lens and the observation prism which are shown in FIG. 8. FIG. 9B is a cross sectional view showing the front lens and the observation prism in a direction orthogonal to a paper surface in FIG. 9A.
As shown in FIG. 8, the conventional operation microscope is constructed so as to observe fundus and its surroundings Er′ of an eye to be operated E in a state where an observation prism 102 is disposed between an anterior focal point F of an objective lens 101 and a front lens 103. Note that the fundus and its surroundings Er′ indicate surrounding regions of a fundus Er of the eye to be operated E. Reference symbol F′ in FIG. 8 indicates an intermediate image plane on which an image of the fundus Er is formed.
As shown in FIGS. 9A and 9B, light beams are refracted on two refractive surfaces 102a and 102b of the observation prism 102 to change traveling directions of those. The two refractive surfaces 102a and 102b each are formed flat (that is, curvature is 0). The observation prism 102 is constructed such that the refractive surface (oblique surface) 102a has a predetermined oblique angle with respect to the refractive surface 102b. Note that the predetermined oblique angle is angle at an intersection (prism apex) 102c of the two refractive surfaces 102a and 102b. 
In FIGS. 8, 9A, and 9B, principle rays R1, R2, and R3 corresponding to observation points of the eye to be operated E are shown. As enlargedly shown in FIG. 9A, when the light beams are transmitted through the observation prism 102, the light beams are significantly refracted on the oblique surface 102a and then refracted on the refractive surface 102b. After that, the light beams are incident on the front lens 103 at a large angle with respect to the optical axis direction of the front lens 103. Therefore, a large aberration is caused in the refracting direction, so that the entrance pupil of the observation optical system located above the objective lens 101 is not clearly imaged on the cornea of the eye to be operated E to be blurred. Although not shown here, an image of the exit pupil of an illumination system for emitting illumination light similarly is blurred.
On the other hand, as shown in FIG. 9B, an aberration is not caused in a direction orthogonal to the oblique direction, so that principle rays R4, R5, and R6 are condensed at a point H.
When an observation is conducted with a less glare state, it is necessary to separate an illumination light flux from and an observation light flux on the cornea of the eye to be operated E. That is, as shown in FIG. 10A, it is necessary to clearly separate images A of the entrance pupils of the observation system from an image B of the exit pupil of the illumination system on a cornea C.
However, according to the conventional operation microscope, as described above, imaging states of the entrance pupils of the observation system and an imaging state of the exit pupil of the illumination system are poor. That is, as shown in FIG. 10B, images A′ of the entrance pupils and an image B′ of the exit pupil are formed on the cornea C with blurred states. Therefore, the images A′ and the image B′ overlap with one another, so that the images A′ and the image B′ are not clearly separated from one another. Thus, the observation light is mixed with a glare, an observation with high visibility cannot be conducted.